Legislation relating to environmental protection puts an increasing restrictions on emissions from combustion engines. For this reason, modern vehicles are equipped with catalysts at which hydrocarbons are converted into water and carbon dioxide. A catalyst must have reached its operating temperature to operate efficiently. Many attempts have been made to reduce the time needed for catalysts to reach their light off temperature. Even though some of these attempts have been successful and the time needed to reach light off temperature has been reduced, starts and, in particular, cold starts are still responsible for a great proportion of total emissions of hydrocarbons from an engine. A significant reduction of emissions of hydrocarbons in the starting process of an engine would therefore generate a significant reduction of the total emissions of hydrocarbons from the engine.
Before the catalyst has reached its operating temperature, the catalyst will not convert hydrocarbons into water and carbon dioxide. It is therefore of fundamental interest to make sure that the amount of injected fuel is kept as low as possible during cold start conditions. However, it is difficult to maintain stable combustions at cold start conditions. To supply conditions that are necessary for stable combustions, normally a surplus amount of fuel is injected during cold start.
With in-cylinder (or direct) injection, an additional problem occurs during the start up phase of the engine. Direct injection engines include a high pressure pump, which operating pressure must be sufficient to atomize the injected fuel. To reach the operating pressure, the engine must run at a considerable speed. When the first injections take place, the engine speed has not been achieved yet. Therefore, the fuel is poorly atomized. To sustain stable combustion, excess fuel is injected. The operating pressure of the fuel pump during the pressure build-up phase during starting is impaired as a consequence of adding additional fuel. Reducing the amount of injected fuel, accelerates pressure build-up upon starting and reduces hydrocarbon emissions.